Ore beneficiation method



'United States Patent Office 2,723,029 Patented Nov. 8, 1955 OREBENEFICIATIN METHOD James E. Lawver, Lakeland, Fla., assignor toInternational Minerals & Chemical Corporation, 4a corporation of NewYork Application April 24, 1951,*Serial No. 222,689

14 Claims. (.C-l. 209-127.)

This invention relates to the beneiciation of multicomponent materials.More particularly, it relates to the electrostatic separation ofmulticomponent ore material. Still more particularly, it relates to atreatment of granular ore feeds'to render separations in theelectrostatic iield more complete and effective.

When beneciating orcs in accordance with the electrostatic methods knownheretofore, a number of deleterious effects appear which make `itdiflcult to obtain purity of products obtainable by other methods suchas flotation. After suliicient passes through an electrostatic field orfields, ore'particles apparently lose the vproper charge -or magnitudeof charge, either becoming passive or inactive, or even exhibiting`random charges, sometimes even of the opposite polarity from the chargeoriginally exhibited. Further, the drying of the ore apparently has amarked -eiiect upon `the ore particles. Indirect-ly heated ore, such asore dried :in an electric oven, exhibits greater selectivity in responseto `an electrostatic eld than ore dried by tumbling in a'rotary kiln, or'equivalent device, in cont-act with combustion products. 'Indirectheating suffers from the disadvantage 'that low heating efficiency doesnot lend itself to economical drying of 'low value, low grade ore. Y

It is an object of Vthe present :invention to overcome disadvantagesandlimitations of the processes heretofore known.

lt is a further object to provide a method wherein ore is maintained atVa high 'level of lsuscepfilzvi'lity to separation by electrostaticfields.

`It is a still vfurtherobject/of the invention to provide a `methodwhereinpart'icles which have lost their `char-ge are .restored `to ahighly `chargeable state.

It is still another object to provide a method wherein ore particles aredried by 'large capacity V.direct heating and the deleterious teiectsovercome/by an after treatment to restore the selectivity of chargingand response to an electrostatic eld.

lt is a still further object to provide a method wherein particlesurfaces are .cleaned and exposed whereby the particles retain high`selectivityrto charging.

It is a still further object to provide amethod which in a few 4passesthrough .electrostatic fields produces concentrates containing only afew percent of foreign matter.

It is a still further object to provide a method for the beneiiciationof phosphate ores to produce phosphatic material concentrates of 74%to`85% bone phosphate of lime (B. P. L.) content l,containing Aonlyapproximately 1% to 4% of insoluble matter.

lt is a still further object to provide a method `for the recovery ofcrystalline products from potash ores, such Vas .sylvinite, which havebecome .electrostatically inactive.

These and other objects will become apparent as the descriptionproceeds.

This invention is based vupon the discovery that ore particles whichhave reached .an inactive stage, .as regards response to electrostaticfields, may be restored to electrostatic separationsusceptibility byexposure `of surfaces 4by impact action. Exposure of surface may bebrought about either by contact between particles or .contact betweenparticles `and mechanical elements, as in grinders, whereby old surfaces'are cleansed and/ or new surfaces .are uncovered.

The impacting action referred to may be illustrated by reference to atype of grinding accomplishable in a hammer mill or impactor. In thistype of grinding, material is put through the mill as rapidly aspossible with a mill setting such that there is a maximum of Aaction atthe surface of the particles and minimum particle disintegration. lIt isnot intended by this statement to rule out particle disintegration bynormal grinding since disintegration does expose new surface areas, butit has been found that such drastic reformation of particles as isaccomplished' in grinding toliberation of components is not necessary inorder to revive the response of the particles to an electrostatic field.The following sieve analysis of an elcctrostatically inactive phosphateore before and after a reactivating treatment in a Ahammer `millillustrates the small amount of reformation necessary.

Asive. Aielve n ysis a ysis Tyler Screen Size Before After GrindingGrinding The above analyses show that only about 4.1% of lines mesh)were produced by passage through the hammer mill. It further shows ageneral reduction in quantity from one mesh size to another. In otherwords, the vaction was one of uncovering clean surfaces rather thanformation -of new surfaces by cleavage, an action which markedly alterssieve analyses and generally produces large quantities of fines.

The novel method of beneficiating ore comprises im pacting concentrateswhich exhibit appreciable reduction in response to an electrostatic eldand subjecting the impacted material to further electrostatic fieldconcentration.

In the over-all process of effecting beneiciation of an ore, the methodcomprises inducing multicomponent dry ore of relatively uniform particlesize selectively to accept an electrical charge, subjecting the chargedore to at least one electrostatic field separation, baring or exposingclean surfaces by predominantly surface attrition action of at least theconcentrate fraction recovered in the separation, and subjecting thematerials after such attrition action to additional separations inelectrostatic elds.

In the preliminary .treatment of ores prior to any passage throughelectrostatic lields, the ore is treated so as .to have a reasonablyuniform particle size. In the case of Florida phosphate pebble ore, theoperation is simply washing to free the phosphate pebbles of clay andsizing to produce Va fraction in the particle size range of -14 mesh and+200 mesh. Florida pebbles of +14 mesh size are washed and sold withoutfurther beneiiciation. If further beneiiciation is required it isnecessary to grind pebbles for liberation. On the other hand, phosphateores from other regions and other nonconductor mineral ores, requiregrinding to the economical liberation stage for freeing Vor unlockingthe ore components.

The material is next dried. This may be accomplished either by direct orindirect heating; for example,`by contact of hot gases with a bed ofgranular material, as in a rotary kiln, fluo-dryer, and the like. Dryore, preferably in a hot condition, is then induced selectively toaccept an electrical charge and the charged particles passed through oneor more electrostatic fields to effect preliminary concentration.

Upon exhibiting a reduction in the response to separation in anelectrostatic field of a tield gradient equivalent to that of the priorseparation in the sequence, the particles are treated to again approachtheir initial activity or respense to strong electrostatic fields. Thistreatment, in the nature of a grinding action, is not to be confusedwith grinding for liberation. The above sieve analyses show thattreatment does not make suflicient change in particle sizes to effectany appreciable liberation. The particles are treated to bring aboutexposure of clean selectively chargeable surfaces, following which theparticles may again be subjected to electrostatic beneciation with orwithout reheating and recharging o the ore particle, depending upon thecharacteristics of the ore being beneciated.

More specifically, feed material, a granular product having a particlesize in the range of approximately -14 mesh and approximately +200 mesh,determined by standard screens, is fed to the initial electrostaticseparations. Charging of the particles preferably is carried out whileparticles are at an elevated temperature in the range of approximately150 F. to approximately 350 F. One method of charging is by conveyanceof particles to a feeder having a grounded contact chute surface ofgraphite, lead, zinc, aluminum, tin, and the like. The charged particlesare then passed between the electrodes of one or more electrostaticseparation units, preferably being fed as freefalling bodies moving in apath normally not in contact with the electrodes.

The strength of the electrostatic Alield which will eieetively alter thepath of particle movement varies with the average particle size of theore fed to the separator and the magnitude of charge on the particle.This may vary from 3,000 volts per inch of distance between electrodesin separating material of relatively fine particle size to 15,000 voltsper inch of distance separating electrodes for beneficiating of coarscrparticles. ln all such discussion of eld strength, it must be borne inmind that corona discharges which ionize air are to be avoided. Ingeneral, it is preferred to operate with a total impressed difference inpotential of voltage in the range of 70,000 volts to 250,000 volts,preferably giving field gradients in initial separations of 3,000 voltsto 10,000 volts per inch, and gradients of 5,000 volts to 15,000 voltsper inch for final separations. This voltage should be maintained at ahigh direct voltage potential substantially free of alternating currentcomponents, i. e., filtered D. C. current should be low in the so-calledA. C. ripple. A steady supply of D. C. voltage may also be obtainedwithout expensive filtering apparatus by the use of such equipment as arectilied radio frequency power supply.

When, upon passage through electrostatic fields of the above indicatedstrengths, the ore particles are rendered inactive with respect to theelectrostatic field, i. e., fail to upgrade upon additional passesthrough an electrostatic field, clean surfaces on particles are exposedby passage through an agitator or impactor; such as, for example, apebble mill, hammer mill, grinder, rod mill, an impactar, screen, or thelike. Following the treatment to clean surfaces, the concentrateparticles may or may not exhibit a restored response to electrostaticfields without being retreated to induce a selective recharging. lf thematerial is to be recharged, it may again be passed over a groundedContact surface such as has been described above. The charged materialthen will be resubjected to electrostatic fields of the strengthhereinbefore described.

The method will be more fully understood from the following descriptiontaken in conjunction with Figures 1 and 2, in which:

Figure l is a diagrammatic flow sheet showing one embodiment of theinvention.

Figure 2 is a graph showing the B. P. L. content of ore fractionseffected by electrostatic separations when orc concentrate has becomeunresponsive to an electrostatic eld and when the ore has beenrejuvenated by the novel method.

Sized ore from a storage hopper 10 is conveyed to a drying station 11.Hot dry ore is then moved by suitable means to a charging unit 12, suchas a grounded vibratory galvanized metal chute. Charged ere issuing fromthe chute passes between electrodes of one or more electrostatic fieldsin series indicated by separation station 13. The tail product 14 isgenerally a discard material but may be reworked by additional passesthrough a tail scavenging section to reduce the content of the desiredproduct component.

The intermediate product 15 is delivered to an impactor 16; for example,a hammer mill. A reformed product may then be treated in a number ofalternating ways7 two of which are illustrated. In the preferred method,the reformed intermediate product is fed directly between the electrodesof one or more electrostatic separation units indicated at station 17.In unit 17, three products are generally produced; a tail 18, which mayor may not be subjected to additional electrostatic processing; aconcentrate 19; and a middling 20, which is recycled to the impactor.

When greater disintegration of particles takes place, as where materialsare prone to fracture, the material may be treated as shown by thedotted line alternative method. In this method, the material from theimpactor 16 is delivered by line 26 to a screening station 21. Material22, larger than some predetermined size which is separated, is recycledby a conveyor, indicated at 22, to the impactor station 16. If anyappreciable quantity of fine material is produced, the fines 24 may beremoved in an air sizer 23. The granular product of the air sizing orthe product from the screening operation 21 are then conveyed with orwithout reheating and/ or recharging to electrostatic separation station17 by means of conveyor 25 for treatment as above described. Theinvention is illustrated by the following example:

Example Florida pebble phosphate was washed and deslimed to produce afraction having material of a particle size in the range of about -14mesh and about +150 mesh. The granular material was dried in a duo-dryerthrough which combustion gases at a temperature of about 325 F. werepassed. The original granular feed had a B. P. L. content ofapproximately 31.5%. The material, while hot, was charged by passagethrough the trough of a galvanized iron vibratory feeder which wasgrounded to the earth by an electrical conductor. The charged particleswere then dropped as free-falling bodies through a series of fourelectrostatic fields having plate electrodes 10 feet in length. Thetotal impressed dilerence of potential was maintained at approximately90,000 volts, and the electrode plates were separated by approximately 9inches.

At the fourth pass (stage 4) through electrostatic fields, anintermediate concentrate or intermediate product was produced which hada B. P. L. content of about 73.7% and about 8% insoluble material andwhich was the feed material fed to electrostatic separations hereinafterdescribed to produce curves A and B of Figure 2. The curves of Figure 2are plots of the B. P. L. content of a concentrate fraction and the B.P. L. content of the corresponding tail fraction produced by anelectrostatic separation at an impressed diterence of potential of90,000 volts and a field gradient of 10,000 volts per inch of distanceseparating electrodes. Curve A shows the results of passing a portion ofthe intermediate concentrate or product obtained from the fourth stagethrough additienal stages of separation without any ,intermediatetreatment. *Lhis `curve shows, .for example, `that the .intermediate`concentra-te from stage 4 gave-a separation in stage 5 in which thefraction collected at the electrode normally attracting the phosphateconcentrate had a B- P- L. .content of 73%; whereas, the vso-called tailfraction had a B. P. L. content `of 73.5%. VFrom this curve it can beseen that added stages of `electrostatic separation are no longer makingeective separations and the concentrate has reached a B. P. L contentwhere `no further upgrading is being accomplished.

A second portion of this intermediate .concentrate obtained from stage 4was y given a surface grind or attrition treatment in a hammer mill.having no screen. The 150 :mesh material fthe fines :produced in thisgrind) was removed Lby screening. |lfihe -35 +150 mesh material,constituting '9,6% of the ,mater-i211 fed to thegrinder (4% finesproduced), -was then reheated to approximately 350 `and'passed-throughone stage `of electrostatic separation in the equipment .used for theypreliminary separations above.

Results .obtained in this separation are shown in curve B of Figure 2.From this curve it can he seen that the B. P. L. of the concentrate feedcan be improved to about the 7.6% to 80% B. L. lhigh `grade .phosphatelevel of commerce with about `1.5% Vto .about 2.2% in soluble matter;whereas, without the intermediate treatment, such products wereunattainable.

For example, from the '73.7% B. P. L. surface ground feed, a concentratewas produced Yin a fifth stage of electrostatic separation having a B.P. L. content of 76.5% and a tail fraction 'having a B. P. L. content of69%. At the sixth stage of electrostatic separation, afconcentrate wasproduced having a B. P. Lsrrntent tof `78.2% and a tail fraction having70.8% B. P. L. The tails from these separation operations are highphosphate content materials of relatively small volume from which thephosphate is recovered by recycling the material to an appropriateintermediate processing stage.

Having thus fully described and illustrated the character of theinvention, what is desired to be secured and claimed by Letters Patentis:

l. A method of beneficiating multicomponent ore which comprises inducingdry ore of relatively uniform particle size and having the componentsthereof substantially completely liberated selectively to accept anelectrical charge, subjecting the charged ore to at least oneelectrostatic field separation, exposing clean surfaces by predominantlysurface attrition action on particles of at least the concentratefraction recovered in the electrostatic separation, and subjecting thematerials after such treatment to at least one additional electrostaticseparation.

2. A method of beneficiating multicomponent ore which comprises inducingdry ore of relatively uniform particle size and having the componentsthereof substantially completely liberated selectively to accept anelectrical charge, subjecting the charged ore to at least oneelectrostatic field separation from which is recovered a concentratehaving substantially no response to further electrostatic fieldseparations, exposing clean surfaces by predominantly surface attritionaction on particles of at least the concentrate fraction recovered inthe separation, inducing the clean concentrate particles selectively toaccept an electrical charge, and subjecting the materials after suchtreatment to atleast one additional electrostatic separation.

3. A method of beneciating multicomponent ore which comprises inducingdry granular ore of relatively uniform particle size and having thecomponents thereof substantially completely liberated selectively toaccept an electrical charge, subjecting the charged ore as free-fallingbodies to the attractive and repulsive forces of at least oneelectrostatic field, collecting at least a concentrate and a tailfraction, grinding lightly at least the concentrate fraction to expose.clean surfaces, and subjecting the materials after such treatment to atleast one electrostatic separation effected by a relatively ,high.impressed .dierence .of potential.

. 4. A method of benelciating multicomponent ore whichcomprises,inducing :dry granular `Vore of relatively uniform particle :size .and.having .the components thereof Substantially completely liberatedselectively to accept an .electrical charge, subjecting the charged oreas freefalliing bodies .to the attractive 'and repulsive forces of atyleast one electrostatic lfield, collecting at least a concentrate and.a tail fraction, grinding lightly at least the concentrate fraction toexpose clean surfaces, subjecting-the material-s .after such treatmenttoat least one electrostatic .separation effected `by a relatively highimpressed ,difference of potential, and collecting at least aconcentrate vfraction of higher B. P. L. content than the impactedintermediate concentrate.

:5. A method-of beneciating phosphate ore which comprises .inducingphosphate ore -of .a particle size in the frange-between about 114 imeshand about 200 mesh, pass- ;ing the charged ore .through vat least oneelectrostatic field as free-falling bodies, collecting at least aconcentrate and a Vtail fraction, the concentrate exhibitingsubstantially no .response Vto an Yelectrostatic field of at least vafield gradient equivalent to that of the prior separations :in thesequence, subjecting at least the concentrate fraction to predominantlysurface attrition action to expose `clean surfaces on particles, andsubjecting the materials after such impact action to at least oneelectrostatic field separa-tion at a relatively high impresseddifference of potential.

|`6. A method 'of `beneficiating phosphate ore which comprises inducingphosphate .ore of a particle size in the range between about 1,4 meshand about 200 mesh selec- 'tively to accept an electrical charge,subjecting the charged ore as free-falling bodies to the attractive andrepulsive forces of at least one electrostatic field having a fieldgradient in the range of about 3,000 volts per inch to about 10,000volts per inch, collecting at least a concentrate and a tail fraction,at least the concentrate fraction of which exhibits substantially noresponse to additional passes through electrostatic fields, lightlygrinding the concentrate fraction in a hammer mill, and subjecting theground concentrate to at least one electrostatic field separation at afield gradient in the range between 3,000 volts per inch and about15,000 volts per inch.

7. A method of beneficiating phosphate ore which comprises inducingphosphate ore of a particle size in the range of about 14 mesh to about200 mesh selectively to accept an electrical charge, subjecting thecharged orc to the attractive and repulsive forces of electrostaticfields until an intermediate concentrate is produced which eX- hibitssubstantially no beneficiation in subsequent electrostatic fields,lightly grinding the intermediate concentrate in a hammer mill, andsubjecting the ground intermediate concentrate to at least oneelectrostatic field separation at a field gradient in the range ofbetween about 3,000 and 15,000 volts per inch of distance separatingelectrodes and equal to the field gradients maintained in preparing theintermediate concentrate.

8. A method of beneficiating multicomponent ore concentrates whichexhibit substantially no response to an electrostatic field followingsubjection to at least one electrostatic field separation whichcomprises grinding lightly the concentrate at least sufficiently toexpose clean surfaces on concentrate particles, and subjecting theimpacted material to further electrostatic field concentration.

9. A method of beneficiating ore concentrates which exhibitsubstantially no response to an electrostatic eld following subjectionto at least one electrostatic field separation which comprises grindinglightly the concentrate at least sufficiently to expose clean surfaceson the concentrate particles, inducing the impacted concentrateselectively to accept an electrical charge, and subjecting the chargedmaterial to additional electrostatic field concentration.

10. A method of beneficiating ore concentrates which exhibitsubstantially no response to an electrostatic field following subjectionto at least one electrostatic field separation which comprises abradingthe concentrate particles at least sufiiciently to expose chargeableclean surfaces, inducing the abraded concentrate selectively to exhibitan electrical charge by exchange of electrons with a surface of low workfunction relative to said ore grounded to the earth by an electricalconductor, and subjecting the charged material to additionalelectrostatic field concentration.

11. A method of beneficiating an electrostatically prepared oreconcentrate which exhibits substantially no response to an electrostaticfield following subjection to at least one electrostatic fieldseparation which comprises grinding lightly the concentrate particles atleast sufciently to expose chargeable clean surfaces, and subjecting thecharged particles to the attractive and repulsive forces of at least oneelectrostatic field having field gradients in the range between about3,000 volts per inch and about 15,000 volts per inch of distance betweenelectrodes.

12. A method of beneficiating an electrostatically prepared oreconcentrate which exhibits substantially no response to an electrostaticfield following subjection to at least one electrostatic fieldseparation which comprises grinding lightly the concentrate particles atleast suiciently to expose clean surfaces, passing the impactedparticles as free-falling bodies through at least one electrostaticfield bounded by electrodes having impressed thereon a difference ofpotential in the range between about 90,000 volts and about 250,000volts, and collecting at least a tail and a concentrate fraction in thevicinity of the lower portions of the electrodes.

13. A method of beneficiating electrostatically prepared phosphate oreconcentrates which exhibit substantially no response to an electrostaticfield following subjection to at least one electrostatic fieldseparation which comprises grinding lightly the concentrate particles atleast sufficiently to expose chargeable clean surfaces, removing nematerial produced from said impacted concentrate, and subjecting thecharged particles to the attractive and repulsive forces of at least oneelectrostatic field having field gradients in the range of about 3,000volts per inch to about 15,000 volts per inch of distance betweenelectrodes.

14. A method of beneficiating electrostatically prepared Florida pebblephosphate ore concentrates of a particle size in the range between about14 mesh and about 200 mesh which concentrates exhibit substantially noresponse to an electrostatic field following subjection to at least oneelectrostatic field separation which comprises lightly grinding theconcentrate particles in a hammer mill and subjecting the groundparticles to the attractive and repulsive forces of at least oneelectrostatic field having a field gradient in the range of about 3,000volts to about 15,000 volts per inch of distance between electrodes.

References Cited in the file of this patent UNITED STATES PATENTS2,197,865 Johnson Apr. 23, 1940 FOREIGN PATENTS 542,988 Great BritainFeb. 5, 1942 OTHER REFERENCES Science, December 7, 1945, vol. 102, pages573-576. Copy in Scientific Library.

1. A METHOD OF BENEFICIATING MULTICOMPONENT ORE WHICH COMPRISES INDUCINGDRY ORE OF RELATIVELY UNIFORM PARTICLE SIZE AND HAVING THE COMPONENTSTHEREOF SUBSTANTIALLY COMPLETELY LIBERATED SELECTIVELY TO ACCEPT ANELECTRICAL CHARGE, SUBJECTING THE CHARGED ORE TO AT LEAST ONEELECTROSTATIC FIELD SEPERATION, EXPOSING CLEAN SURFACES BY PREDOMINANTLYSURFACE ATTRITION ACTION ON PARTICLES OF AT LEAST THE CONCENTRATEDFRACTION RECOVERED IN THE ELECTROSTATIC SEPARATION, AND SUBJECTING THEMATERIALS AFTER SUCH TREATMENT TO AT LEAST ONE ADDITIONAL ELECTROSTATICSEPARATION.